The present invention relates to a mass flowmeter converter and, more particularly, to a converter which is adapted for use in a low detection sensitivity mass flowmeter such as a straight tube type Coriolis flowmeter, and which can measure a Coriolis force acting on a flow tube of a Coriolis flowmeter, which is proportional to mass flowrate, as a time difference at high sensitivity with no transit-time change due to a drift of an operational circuit.
A Coriolis flowmeter is a well known mass flowmeter which is based upon the fact that when fluid flows in a flow tube supported at both ends on supporting members and said tube is driven with an alternate, oscillation at its center portion in the direction perpendicular to its axis, a phase difference is produced between two symmetrically opposite positions on the flow tube and the phase difference is proportional to a mass flow rate. In practice, a driving coil to be excited by a drive circuit is provided in the center of a flow tube supported at both ends on supports and two detecting coils are arranged at symmetrically opposite positions between the center portion and both ends of the flow tube. A signal of a phase difference proportional to a mass flow rate is produced by the action of a Coriolis force and is detected. A mass flow is determined from the phase difference value. If a driving oscillation frequency is supposed to be constant, a phase difference signal can be detected as a time difference signal obtained when the flow tube passes a standard line at symmetrical positions.
When a flow tube supported at both ends on supports is driven with an alternate natural oscillation frequency at its center portion in the direction perpendicular to its axis, a constant driving frequency which corresponds to a size and material of the flow tube and a density of the measurable fluid is obtained at a small driving energy. Therefore, the fluid density corresponding to the driving frequency is determined. For this reason, it is usually adopted to drive the flow tube at the natural oscillation frequency.
A circuit for driving the flow tube at the natural oscillation frequency is a positive feedback circuit that controls an input signal at a constant level by inputting a sine-wave signal output from a detecting coil into a driving circuit.
Accurate measurement of a mass flow by a Coriolis mass flowmeter thus constructed depends upon stable and accurate measurement of a time difference signal. The time difference is measured by counting clock pulses at a specified frequency during the time difference. For instance, in case of a straight tube type Coriolis flowmeter having a high rigidity, a phase difference signal produced by the action of a Coriolis force is small. Therefore, a time difference value of the time difference signal proportional to the phase difference signal is correspondingly small. Such a small time difference may be detected at an accuracy necessary for further measurement in the Coriolis mass flowmeter by using a clock pulse generator of 100 MHz or similar level, which is, however, expensive to use. To stably measure a small value of time difference at a high accuracy, there is still a problem concerning stability of the time difference detecting circuit itself, e.g., the occurrence of a zero drift may reduce detection accuracy. Furthermore, the Coriolis flowmeter itself may be affected by an expansion of measurable fluid due to temperature change.